A large and fast growing area in the field of printing is known as flexography or flexographic ink printing. Flexographic ink printing is a branch of the rotary typographic printing in which the printing is applied to a print-receiving substrate by use of a flexible relief plate with highly fluid, volatile inks which dry rapidly by evaporation. The process was orginally limited to printing paper bags but many inroads have been made in recent years to extend the process to printing various flexible packaging, textiles, newsprint, and other print-receiving substrates.
A flexographic printing ink generally contains three basic types of ingredients denominated as a solvent, a colorant, and a binder. Minor amounts of other additives also may be employed to provide the desired rheology, viscosity, etc. for the printing ink composition.
In recent years, increased emphasis has been placed on the use of water as the solvent, rather than an oil-based solvent. As colorants, dispersed dyes, pigments, or combinations of the two generally are employed.
The binder component of the printing ink composition generally comprises a resin which functions primarily to increase adhesion of the pigment to the substrate, and also often act as a dispersing medium and carrier. Binders heretofore used in water-based printing ink compositions have included acrylic resins. Amine salts of lignins also have been employed as binders in water-based, carbon black colorant
Because acrylic resins are relatively slow drying, their use as binders creates a tackiness, or stickiness, of the printing ink and a material buildup on the second and subsequent impression roll of the printing press, necessitating periodic shutdown for cleaning of the printing equipment with resultant lost time in commercial operations. Although lignin amine salts have reduced tackiness and stickiness as compared to the acrylic resins, their dark color precludes their use in lighter colorant shades, other than the carbon black inks.
Minor amount additives in the printing ink compositions generally may comprise surfactants, slip agents, catalysts, and rheology control agents. Typical surfactants are organic sulfonates, whiles waxes serve as slip agents. Drying agents may comprise lead-derived drying oils, while clays and certain high molecular weight polymers may be employed to control the rheology of the ink composition.